1. Technical Field
The present invention relates to a stacker for stacking and holding a multiplicity of printed printing papers delivered from a printer.
2. Background Art
A stacker adapted to be arranged adjacent to a paper delivery port of a printer, for sequentially taking in a multiplicity of printed printing papers (or sheets) delivered from the printer and for stacking and holding the papers thus taken in on a tray and in order, is known. In this type of stacker, in order to smoothly take in and carry a large number of papers delivered at a predetermined rate from the printer, a paper carrying speed in a stacker body is normally set at a paper intake speed (equal to the paper delivery speed of the printer) or higher. Especially, in order to shorten a time required for taking in the paper and stacking it on the tray, the conventional stacker has set the paper carrying speed at a sufficiently high level as compared with the paper intake speed.
Under this speed setting, when the stacker is used for a high-speed printer (referred to as, e.g., a high-speed page printer) capable of printing dozens of papers per minute, the paper carrying speed tends to reach a high level such as several hundreds of mm/s. The papers carried at high speed are discharged from an outlet port provided on the back side of the stacker to fly above the tray extending rearward of the stacker, and freely fall on the tray to be stacked one by one. The tray is usually provided with a paper support surface inclined in such a manner that the proximal end thereof adjacent to the stacker body is positioned lower than the distal end thereof. Thus, the paper that has first fallen on the tray slides downward on the support surface by its own weight, and bumps against a back plate of the stacker body to come to a stop at a predetermined position. The paper that has subsequently fallen on the tray slips downward on the first paper, and it also bumps against the back plate of the stacker body to come to a stop at a predetermined position. In this way, a multiplicity of papers are stacked with the edges thereof aligned with one another.
Incidentally, if a speed at which the paper is discharged from the outlet port is excessively high, the flying distance of the paper may be increased and thus the sliding distance on the tray may be increased. In this case, it is often difficult to stack a large number of papers in a mutually exact alignment on the tray. In view of this, in the conventional stacker adapted to be used for the high-speed printer, a paper discharge speed is set lower than a paper carrying speed, so as to prevent the excessive flying of the paper. FIG. 13 is a velocity diagram showing one example of a paper-transfer velocity change between the paper intake and the paper discharge in the conventional stacker used for the high-speed printer. In FIG. 13, a horizontal axis represents a time (ms) and a vertical axis represents the peripheral velocity (mm/s) of an eject roller installed at the outlet port of the stacker. The eject roller is one of drive rollers for transferring the papers delivered from the high speed printer inside the stacker body. The rotational speed of the eject roller is controlled by stages over a time period T1 for taking the paper into the stacker body, a time period T2 for carrying the paper in the stacker body and a time period T3 for discharging the paper above the tray.
In the illustrated example, when the stacker receives a paper delivery signal from the high-speed printer, the eject roller first starts to rotate at a peripheral velocity of 180 mm/s identical to the paper delivery speed of the printer (the intake time T1). The paper is transferred at this intake speed v1 to the eject roller along a paper guide in the stacker body. When the paper arrives at a predetermined position at [t01], the eject roller increases the peripheral velocity to 750 mm/s, and catches the paper to transfer it to the outlet port at this carrying speed v2 (the carrying time T2). When the leading portion of the paper protrudes from the outlet port by a predetermined length at [t02], the eject roller decreases the peripheral velocity to 400 mm/s, and acts to discharge the paper from the outlet port at this discharge speed V3 above the tray to make it fly thereabove (the discharge time T3). After that, at [t03], the eject roller is returned to the intake speed V1 and waits for the next paper.
Also for the next paper, the peripheral velocity of the eject roller is controlled in the same way, i.e., to the intake speed v1=180 mm/s from t03 to t11, to the carrying speed v2=750 mm/s from t11 to t12, and to the discharge speed V3=400 mm/s from t12 to t13. Subsequently, the printed papers are discharged successively from the printer at the speed of, e.g., 42 sheets per minute (the cycle of 1430 ms). Thus, the eject roller changes the peripheral velocity thereof by stages in the same way, and repeats the intake, carry and discharge of the paper, so as to stack a large number of papers on the tray.
The above-described stacker for the high-speed printer is required to be able to automatically and accurately stack and hold several thousand papers on the tray, so as to cope with the automatic operation of the printer. For this purpose, this type of stacker generally incorporates therein an automatic lift mechanism of the tray. The automatic lift mechanism includes a paper stack sensor of a reflection type arranged at a predetermined position on a back plate below the outlet port of a stacker back-side, and a tray drive mechanism activated in accordance with a sensing signal of the paper stack sensor. When a paper stack piled on the tray closes the front side of the paper stack sensor, the paper stack sensor outputs the sensing signal, and thereby the tray drive mechanism is activated to move the tray downward. Consequently, the position of the top surface of the paper stack on the tray is held at a substantially constant height in the vicinity of the paper stack sensor. When the tray moves downward in accordance with the increase in the number of stacked papers and reaches a lowest position with several thousand (e.g., three thousand or more) papers being piled thereon, a limiter placed at the lowest position of the tray detects the tray and outputs a stop signal to the control section of the printer and stacker. As a result, the printer and stacker come to a stop, and thereby a printing and paper stacking operation is completed.
In the conventional stacker having the above configuration, a circumstance may arise wherein a paper, expected to fly and fall toward the tray, falls, contrary to expectations, along the back plate of the stacker body while coming into partially contact therewith. In this case, the paper cannot be laid in flat on the tray or on the previously formed paper stack, but tends to lean at a part thereof against the back plate of the stacker body.
One of the factors of this falling behavior of the paper is an adsorbing action due to static electricity charged on the paper in the printer. If the paper is considerably charged, the paper cannot sufficiently fly after it is discharged from the outlet port of the stacker, but is adsorbed to the back plate of the stacker due to an electrostatic absorbing force. Therefore, in order to eliminate the static electricity charged on the paper, the conventional stacker is constituted such that a charge-eliminating brush is placed around the outlet port and the paper is discharged above the tray from the outlet port after any charge is eliminated by coming into contact with the charge-eliminating brush. The charge-eliminating brush can exhibit a charge-eliminating effect to some degree by optimizing the dimensions and location thereof, but is difficult to perform perfect charge-elimination, and thereby the paper is actually discharged with the static electricity of 1 kV to 2 kV remaining on the paper. An effective method for completely eliminating the static electricity of the paper is in using a voltage-impress type charge-eliminator to generate ions through a corona discharge and to neutralize the charge of a charged object by the ions. However, this method is generally expensive and is thus difficult to employ.
FIG. 14 is an enlarged view of a part of a conventional stacker around an eject roller, for illustrating the leaning phenomenon of a printing paper. A top paper 3 of a paper stack 2 piled on a tray 1 is placed in such a manner as to lean at an edge portion thereof against a back plate 4 of a stacker body. In this state, if the leaning portion of the paper 3 closes the front side of a reflection-type paper stack sensor 5, the paper stack sensor 5 outputs a sensing signal and thereby the tray 1 moves downward in spite of the fact that printing papers are not stacked to a predetermined height. In the case where the paper 3 is adsorbed to the back plate 4 and keeps on closing the front side of the paper stack sensor 5 even after the tray 1 moves downward, the tray 1 continues to fall and finally arrives at a most lowered position. Then, a limit switch (not shown) is turned on and the printer and the stacker come to a stop, despite the fact that the paper stack has not reached a predetermined amount, which results in a loading failure.
It has been found that the paper 3 is liable to lean against the back plate due to electrostatic absorption, especially in the case where the end portion of an eject roller guide 7 arranged around an eject roller 6 of the stacker is exposed as a part of the back plate 4 of the stacker body, as shown in FIG. 14. In this case, the exposed portion of the eject roller guide 7 also acts as the exterior part of the stacker body and, therefore, the eject roller guide 7 is formed from a surface-treated steel plate having a very-thin oxide film with a thickness on the order of xcexcm. As a result, when the charged paper 3 comes into contact with the eject roller guide 7, the charge of opposite polarity to the paper 3 is induced in the contacted portion of the surface-treated steel plate, and the induced charge is difficult to discharge due to the oxide film, which results in the condition where the paper 3 is liable to be adsorbed electrostatically.
Another factor in the leaning phenomenon of the paper against the back plate of the stacker body is the fact that the paper delivered from the stacker has a curled configuration. The paper printed in a high-speed printer is normally delivered through a process of fixing a toner on a paper surface by heat, and therefore is generally deformed and curls after it is delivered. This deformation makes the outer edge of the paper rise upward when the paper is placed on a flat surface with the printed side thereof facing upward, and this is referred to as a concave-curl in this specification. Every paper delivered from the printer is taken into the stacker, while exhibiting a concave-curl shape, in a face-up state where the printed side of the paper faces upward.
In the case where the stacker carries the paper taken-in from the printer and discharges it above the tray in the face-up state, the paper gains the concave-curl shape as it is fed out from the outlet port, and thereby the leading end portion of the paper is raised. Especially in the case where the carrying speed of the eject roller is high, the leading end portion of the paper is further curled upward due to a lifting force. As a result, the instant that the rear end of the paper leaves the eject roller and the paper is released from the discharge force of the eject roller, the paper stalls and falls down without flying. In this situation, if the residual charge remains on the paper, the paper is adsorbed onto and leans against the back plate.
A more detailed explanation will be given with reference to FIG. 13. The time period concerned with the flying behavior of the paper consists of the carry time T2 (ti1 to ti2; i=1 to 3000) and the discharge time T3 (ti2 to ti3; i=1 to 3000), and particularly the carrying speed V2 for the carry time T2 significantly affects the flying mode of the paper. This is because the paper transfer mechanism of the stacker is so constructed that a major part of the paper (e.g., five-sixths of a A4-sized paper measured along a feeding direction) is fed out above the tray from the outlet port during the carry time T2 and the remaining part (one-sixth of the A4-sized paper measured along the feeding direction) is carried and discharged during the discharge time T3.
It is generally difficult for the stacker to correct therein the concave curl of the paper provided in a printing process. In order to correct the concave curl, it is advantageous that a curl-eliminating unit (a decurler) is installed between the printer and the stacker to eliminate the curl of the paper delivered from the printer before it is taken into the stacker. However, the decurler is generally expensive and requires adjustment according to the degree of the curl and, therefore, it is difficult to employ.
On the other hand, a stacker having the function of inverting the paper during a carrying stage is known. This stacker can turn the paper taken-in from the printer upside-down in the carrying stage and can discharge the paper above the tray in a face-down state where the printed side of the paper faces downward. In this case, when the paper has gained the concave-curl shape as it is fed out from the outlet port, the leading end of the paper drops down toward the tray, so that, even in the case of a high carrying speed, the paper can sufficiently fly without being inconveniently affected by a lifting force, whereby the problem of paper leaning against the back plate does not always occur. However, a carrying mode including paper inversion requires a longer time than a normal face-up carrying mode, and therefore, even a stacker having a paper inverting function is generally so constructed that the face-up carrying mode can be optionally selected.
The object of the present invention is to provide a stacker which, even when a paper having a concave-curl shape taken-in from a printer is carried and discharged in a face-up state above a tray, can prevent the paper from leaning against a back plate of a stacker body and can solve the problem of a possible loading failure of papers during an automatic operation.
In order to accomplish the above object, the present invention provides a stacker comprising a housing provided with an inlet port and an outlet port; a tray arranged beneath the outlet port of the housing in an upward/downward movable manner; a transfer mechanism for transferring printed sheets from the inlet port to the outlet port and discharging the sheets above the tray through the outlet port; and a control mechanism for controlling a sheet transfer velocity of the transfer mechanism in three stages including an intake speed at which the printed sheets are taken in through the inlet port, a carrying speed at which the sheets are carried from the inlet port to the outlet port and a discharge speed at which the sheets are discharged from the outlet port above the tray, wherein the carrying speed is higher than the intake speed and the discharge speed is not lower than the carrying speed.
In a preferred embodiment, the carrying speed is not more than 500 mm/s.
Also, in a preferred embodiment, the discharge speed is not more than 500 mm/s.
It is preferred that the transfer mechanism includes a pickup roller arranged at the inlet port, a first drive source for the pickup roller, an eject roller arranged at the outlet port and a second drive source for the eject roller, and that the control mechanism controls the second drive source in three stages of the intake speed, the carrying speed and the discharge speed.
The tray may be provided with an inclined support surface of which a proximal end, nearer to the housing, is positioned lower than a distal end.
The stacker may further comprise an automatic lift mechanism for moving the tray downward as the number of printed sheets stacked on the tray increases.
Also, the stacker may further comprise an insulating member arranged on a part of a lateral support surface of the housing located below the outlet port.
In this arrangement, it is advantageous that the insulating member is formed from a thin plate secured to the lateral support surface.
The present invention further provides a stacker comprising a housing provided with an inlet port and an outlet port; a tray arranged beneath the outlet port of the housing in an upward/downward movable manner; a transfer mechanism for transferring printed sheets from the inlet port to the outlet port and discharging the sheets above the tray through the outlet port; and an insulating member arranged on a part of a lateral support surface of the housing located below the outlet port.